Relative humidity
How much moisture the air holds compared to the most it could hold at that temperature, shown as a percentage.
Relative humidity (RH) is the percentage of water vapor in the air relative to the maximum the air can hold at its current temperature. Warm air holds more moisture, so the same amount of water gives a lower RH when it's warm and a higher RH when it cools — which is why moisture condenses on cold windows. For indoor comfort and health, the EPA recommends keeping RH below 60%, ideally between 30% and 50%. A cheap hygrometer measures it. See [why your house feels humid](/guides/why-your-house-is-humid-and-how-to-fix-it).